In recent years, thin coatings of certain species of metal chalcogenides, such as titanium sulfide (TiS.sub.2) and other transition metal sulfide materials have been intensively investigated. Such coatings are useful, for example, in lithium battery electrodes and solar control panels. Coatings of TiS.sub.2 may be formed using chemical vapor deposition (CVD) methods, by sulfurization of titanium metal at elevated temperatures, and by sputtering methods.
In order to prepare TiS.sub.2 thin coatings by plasma chemical vapor deposition methods (PCVD), a titanium source, such as titanium tetrachloride (TICl.sub.4) or Ti metal, is reacted with hydrogen sulfide (H.sub.2 S) in a plasma between 300.degree. and 450.degree. C. This results in either powder formation or deposition of thin coatings, depending on the conditions. As exemplified by Kikkawa et al (APPL. PHYS. A 1989, 49, 105) gaseous TICl.sub.4 and H.sub.2 S were reacted for 10-60 minutes in a low pressure glow discharge at temperatures below 450.degree. C. to yield a thin coating of TiS.sub.2. However, such techniques require that relatively cumbersome, expensive apparatus be used to generate the exacting low pressure experimental conditions.
In order to fabricate TiS.sub.2 coatings by low pressure chemical vapor deposition (LPCVD), gaseous TiCl.sub.4 is reacted with H.sub.2 S in a nitrogen or argon stream at low pressures (.ltoreq.30 torr). Representative of LPCVD techniques is Kanehori et al. (J. ELECTROCHEM. SOC 1989, 136, 1265), in which TICl.sub.4 and H.sub.2 S were reacted in the gas phase at 510.degree. C. to produce stoichiometric coatings of TiS.sub.2. However, a major drawback of such techniques is that the deposition rates are quite slow. For example, deposition rates may vary between 3 and 9 microns per hour, depending upon the carrier gas flow rate.
A study of the formation of TiS.sub.2 coatings by atmospheric pressure chemical vapor deposition (APCVD) was reported by Motojima et al (BULL. CHEM. SOC. JPN. 1978, 51, 3240), in which a gaseous mixture of TiCl.sub.4 and H.sub.2 S in argon yielded TiS.sub.2 thin coatings at temperatures between 400.degree. and 850.degree. C. However, the crystallinity, stoichiometry and resultant coating density varied markedly with temperature and flow rate. As a result, this technique makes it difficult to reproduce thin coatings of given characteristics with any reliability.
In such prior art methods, various problems are encountered. Relatively complex equipment and instrumentation are required to prepare the coatings. Additionally, use of a toxic and extremely odiferous gas, such as H.sub.2 S, is necessary. Also, the TiS.sub.2 coating stoichiometry can vary significantly from the desired titanium to sulfur ratio of 1:2. Further, the coating deposition rates of the prior art methods tend to be quite slow, especially where thin coatings of high quality are desired. Moreover, the density of the resultant TiS.sub.2 coatings is likely to be less than that of bulk TiS.sub.2. This may consequently lead to inferior electrical, optical and diffusion properties.
U.S. Pat. No. 5,112,650, which is herein incorporated by reference, discloses preparation of metal chalogenide coatings by employing separate reactant streams of vaporous metal halide and a source of vaporous chalcogen, preferably an organic thiol. This process results in rapid deposition, but requires monitoring of two separate reactant streams as well as thorough mixing of the streams prior to contact with the substrate.
The preparation of titanium disulfide coatings from a single source precursor prepared by the reaction of titanium tetrachloride with two equivalents of an alkanethiol is reported by Winter et al in INORGANIC CHEMISTRY (1993), pages 3807-3808, and is also the subject of U.S. Pat. No. 5,298,295, which is herein incorporated by reference. However, the use of alkanethiols other than cyclic alkanethiols resulted in complexes of exceptionally high volatility, which made their manipulation and characterization difficult. Moreover, some of the complexes were unstable to the extent that in certain solvents they reacted to form polymeric compounds having the empirical formula TiCl.sub.2 S.